git cl: cleanup branch config get/set/unset.

Previously there was a soup with add-hoc formatting with
current branch name, which wasn't always set (see bug 611020).
This CL makes sure all such operations now:
 * properly use types --int and --bool
 * go through the *only* appropriate get/set/unset function.

Furthermore, tests were a mess wrt to raising exceptions when
git processes terminated with an exception. This CL cleaned up,
though I didn't go through all expectations, so some returns of
empty stdout instead of raising CalledProcess error are likely
remaining.

Disclaimer: this CL is not necessarily fixing the referenced bug
below, but it should at least provide better stacktrace when
the bug manifestst itself.

BUG=611020
R=agable@chromium.org

Review-Url: https://codereview.chromium.org/2259043002